The present invention relates to an electrophotographic apparatus comprising an improved exposure means.
In a typical electrophotographic apparatus such as an electrostatic copying machine a photoconductive member such as a drum is uniformly charged and radiated with a light image of an original document to form an electrostatic image through localized photoconduction. A toner substance is applied to the drum to develop the electrostatic image into a visible toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
Various conditions can cause blank background areas of copies to print gray rather than white. One cause is that the original document has a colored, rather than a white background. Numerous office documents such as slips are blue, pink, etc. in color. Another cause is variations in the operation of a corona charging unit due to variations in temperature, humidity, supply voltage and the like. Either condition will cause the electrostatic potential of background areas of the electrostatic image on the drum to have a value which differs from the desired value. In the case of colored documents, due to the spectral sensitivity characteristics of known photoconductive substances, photoconductive drums are generally less sensitive to colored light than to white light. Thus, the electrostatic potential of background areas of electrostatic images of colored documents will be excessive and the background areas will print gray rather than white. In the case of variations in the charge intensity of the drum prior to exposure, the background areas of the electrostatic image may have too high or too low potential. Excessive potential causes gray background areas. Insufficient potential causes the dark areas to appear washed out.
Such a problem may be overcome by varying the intensity of the light image in accordance with the electrostatic image potential. In the case of colored documents, an exposure increase will cause the background areas to print white as desired. For every value of electrostatic background area potential within the practical limitations of the apparatus there is a value of light image intensity which will produce a proper copy. However, electrophotographic apparatus known heretofore has not been available which is capable of satisfactorily controlling light image or exposure intensity in accordance with eletrostatic image potential.